Drainback Solar Hot Water Collection Innovations
By Peter Biondo
D
rainback methods for solar hot water heating are not new to the solar industry. However, drainback technology has changed over the years to include
innovative features from design installers all over the United States. In this article, I will discuss these innova- tions which are widely and successfully used by licensed design installers who prefer the service-friendly qualities that drainback systems tend to offer. In my own experi- ence, designing for drainback is an application-driven decision. In a drainback system, the collectors are in fail- safe mode — always completely empty except for cycles of solar collection. Ordinarily, the collectors and the col- lector piping slope to drain into a reservoir within the building envelope — most likely in the mechanical room. The other form of solar collection is the solar closed loop, in which there is a pressurized antifreeze loop between the collectors and the heat exchanger. These are very dif- ferent collection systems. The use of drainback is found to be preferable when hot
water loads (demand) have wide variations. This is often determined by the occupancy of the building (for domes- tic hot water) or seasonal variations (for space heating). The all-important specification to consider for drainback is that the collectors and piping must slope to drain back to the drainback reservoir (there are ways around that — I will point them out later). Roof-mounted solar collectors offer this advantage to drainback. The advantage of the closed loop system is that the collectors may be mounted anywhere in relation to the solar storage tank. There is a great deal of debate over which system is better than the other, and I have listened to both sides. I personally rec- ommend understanding the benefits and disadvantages of both systems and then choosing the one that most appro- priately matches the building and the application. This article will focus on drainback solutions. The most significant advantage of drainback design is built-in freeze protection with the added benefit of col-
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lector overheating protection. An empty collector won’t freeze at night or during cloudy days with freezing tem- peratures. When the solar tank reaches its high limit tem- perature during a collection cycle, collection ends, circu- lation ceases, and every bit of working fluid drains out of the collectors. Most solar thermal collectors, without working fluid, cannot be damaged by sitting in the sun in the heat of the day. By design, drainback requires that solar thermal collectors be emptied by gravity. Most domestic flat plate manufacturers internally pipe
collectors in what is known as a harp design, with high and low manifolds connected with vertical risers. These collectors will typically drain empty, as long as the man- ifolds are sloped to drain. Many European collector mod- els have what is referred to as a serpentine pipe configu- ration. Serpentine collectors may drain by gravity, while others that cannot are designed exclusively for closed loop systems (see Figure 1). Consult the manufacturer about a drainback option for serpentine models. Few heat pipe evacuated tubes can be integrated for drainback. The manifold must be drainable, and the tubes able to reach stagnation without compromising the vacuum seal. Consult the evacuated tube manufacturer for the drain- back option. For buildings such as schools, seasonal lodges, second
homes and offices that are unoccupied for days, weeks, or even months at a time, drainback should always be given serious consideration. This is also the case for space heat- ing systems where the collectors would not be providing for a summertime load. Drainback systems do not require a heat dump, which would be necessary for some antifreeze closed loop systems. In a closed loop system, excess solar energy must be dumped when chronic and excessive temperatures are anticipated in the glycol loop. That would be a requirement for the applications listed above and especially for space heating. However, there are multitudes of domestic hot water and space heat- ing/pool heating/reheat combinations where the solar Continued on page 48
March 2011
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